In heat-sensitive recording materials, the image and non-image areas are represented as the distribution of temperature difference. For visualization of this distribution have been devised various methods utilizing, e.g., fusion or sublimation transfer of coloring materials, coloring reaction between two components by hot fusion or destruction of capsules, and changes in optical characteristics by phase transition. The heat recording materials of this kind can provide recorded images in a dry process using a simple system, and have an advantage of requiring no maintenance. Therefore, they have been widely used as output materials of various kinds of printers, word processors, facsimile and so on. Further, with the recent much progress in laser recording apparatus, the application of the aforesaid heat recording materials to optical disks and platemaking materials for photomechanical process has been examined.
Hitherto, silver halide photosensitive materials requiring wet processing operations have been employed as platemaking materials. In view of requirements for simplifying processing steps and problems of environmental pollution by processing solutions, however, the development of dry processes has been desired, and some technological proposals on heat-sensitive recording methods have been produced recently. In particular, the recording of images by laser beams is advantageous from the viewpoint of resolution. For instance, the system referred to as dye ablation utilizing high output laser has been developed. The recording materials for such a system are disclosed in JP-A-7-164755, JP-A-7-149063 and JP-A-7-149065, and the image forming devices therefor are disclosed in JP-A-8-48053 and JP-A-8-72400 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In such a system is used a recording material having a support coated with a dye composition comprising an image dye, a substance having its absorption in the wavelength region of laser (an infrared absorbing substance) and a binder, and an image is recorded on the recording material by irradiation with a laser beam from the dye coating side. When a laser beam strikes a spot on the recording material, the image forming layer corresponding to the spot undergoes an abrupt local change due to the energy conferred thereon by the laser beam, and thereby the substances are expelled out of the layer. According to the references cited above, the change caused therein is not a perfectly physical change (e.g., fusion, evaporation or sublimation), but a certain chemical change (e.g., bond rupture), and thereby the image dye is removed perfectly but not partially. The dye ablation system as mentioned above has disadvantages in that high output laser is indispensable in order to elevate the efficiency of dye removal at the spot struck by a laser beam and it is required to have a dust catcher installed in order to collect the dye removed.
With respect to the system requiring no dust catcher, U.S. Pat. No. 5,171,650 discloses the image recording method of ablation transfer type wherein laser is used as heat source. In this system, a dye donor sheet having a dynamic release layer provided with the topcoat of an ablative carrier is used, and the image is transferred onto a separate dye-receiving sheet placed so as to adjoin the dye donor sheet and be in a proper position. Therefore, this system has a drawback that the donor sheet useless after image recording must be disposed of as a waste. For this system also, high output laser is indispensable in order to heighten the transfer efficiency. Accordingly, the hitherto known heat-sensitive recording systems utilizing the ablation by laser have problems that they require high output laser and the generation of dust and waste can hardly be avoided.
As another laser-utilized system of heat-sensitive recording, although no ablation accompanies the recording, JP-A-6-194781 discloses a system into which the system referred to as "dry silver" is developed. In this system, the recording with laser is performed on a recording material comprising a silver source capable of being reduced thermally, a silver ion reducing agent and a photothermic conversion dye. However, this system is practically insufficient in storability of non-image area and heat sensitivity.
As for the other laser-utilized heat-sensitive recording systems, the compounds changing their absorption by pyrolysis of carbamates are disclosed in U.S. Pat. Nos. 4,602,263 and 4,826,976, and the compounds developing yellow color by the pyrolysis of t-butoxycarbonyl group introduced to the hydroxyl groups therein are disclosed in U.S. Pat. No. 5,243,052. Those recording. systems utilize an irreversible monomolecular reaction, and are suitable for very short-time recording of images by the use of laser. However, they are insufficient in sensitivity, so that it has been expected to increase their sensitivities.
As a means to elevate the photosensitive speed of photoresist by leaps and bounds, on the other hand, the compounds embodying a new concept that an acid catalyst is multiplied by the action of an acid catalyst are disclosed in JP-A-8-248561. The acid-multiplied compounds described in this reference include the compounds producing organic acids by electron transfer subsequent to the decomposition of t-butyl esters or the like by an acid catalyst, the compounds producing organic acids by B-elimination subsequent to the acid catalytic decomposition of acetals or ketals, and the compounds producing organic acids by pinacol rearrangement in the presence of an acid catalyst.
By combining these compounds each with a compound changing its absorption characteristics by the action of an acid, highly sensitive color image-forming recording materials can be obtained. Further, such compounds have a function as an acid generator, and generate acids by the action of heat at the time of recording images in laser heat mode. Therein, it is unnecessary to add other special acid generators.
However, those compounds are practically insufficient to ensure both sensitivity and storage stability for the recording materials of color image formation type. Further, they generate stain or gas upon decomposition. Therefore, it will be necessary to introduce improvements in those compounds if their application to the recording materials of color image formation type is intended. In addition, it is not so easy to synthesize those acid multipliers, so that further improvement is required for them in this respect also.
Additionally, 1-methanesulfonyloxy-5-(.alpha.-tetrahydropyranyl)oxypentane, which is included in the structures of acid generators according to the present invention, is described in Journal of Organic Chemistry, vol. 53, pp. 318-327. Therein, however, that compound is described simply as a synthesis intermediate of 5-[(.alpha.-tetrahydropyranyl)oxy]-1-iodopentane, and the application of the compound as an acid generator to image recording media is mentioned nowhere.